A variety of electron microscopes and associated surface analyzers have evolved in recent years. General background is given, for example, in Introduction to Analytical Electron Microscopy, Plenum Press (New York 1979). A popular type is a scanning electron microscope in which a focused electron beam is scanned over a sample surface with secondary electrons being detected in correlation with scanning position and processed electronically to provide a picture of topographical features. Associated mapping of chemical constituents in the surface is achieved with characteristic X-rays produced by the electron beam. However, resolution from the X-rays is not commensurate with the topographical resolution. Also, X-rays are not suitable for detecting elements with low atomic numbers or for near surface sensitivity.
Another method for analyzing surfaces is with secondary Auger electrons generated at the sample surface by the focused primary electron beam. Auger microprobes are suitable for detecting elements with low atomic numbers and have sensitivity to a few atomic layers. Surface mapping of elements is accomplished by scanning with the primary electron beam. An example of a scanning Auger microprobe is provided in U.S. Pat. No. 4,048,498 (Gerlach et al). Scanning Auger is limited in analysis area to about 500 angstroms diameter by scattering of the primary beam in the surface region.
Another approach to surface analysis is electron spectroscopy for chemical analysis (ESCA) which involves irradiating a sample surface with X-rays and detecting the characteristic photoelectrons emitted. The photoelectrons are filtered by electrostatic or magnetic means which allow only electrons of a specified energy to pass through. The intensity of the resulting beam reflects the concentration of a given chemical constituent of the sample surface. U.S. Pat. Nos. 3,617,741 (Siegbahn et al) and 3,766,381 (Watson) describe such a system. Chemical mapping of the surface requires moving a component or aperture to detect electrons from various parts of the surface, since X-rays generally cannot be focused sufficiently onto small areas of the surface to allow scanning with high resolution.
Therefore, continuing efforts have been directed toward direct imaging of characteristic emissions. One approach is described in "Photoelectron Microscopy--Applications to Biological Surfaces" by 0. Hayes Griffith, presented at a symposium "Small Area Solid and Surface Analysis" in New Orleans, Feb. 25Mar. 1, 1985. The system decribed therein images low energy photoelectrons from ultra-violet radiation. It is acknowledged therein that an elemental analysis is not provided. Also, imaging with higher energy electrons has been less successful because of aberrations that become more prominent.
The Griffith document, on Page 16, describes a further approach in which emitted electrons are focused with spiral tragectories along magnetic flux lines. Resolution depends on the diameter of the spiral, and it is pointed out that the main limitation is that maximum magnification, and therefore resolution, is low. Energy filtering of the electron beams is important to obtain a monochromatic beam characteristic of an element being analyzed. Electrostatic filtering may be achieved with concentric hemispherical conductors having an applied voltage therebetween, such as described in aforementioned U.S. Pat. No. 3,766,381.
A magnetic system for filtering electrons is described in "Modification of a Transmission Electron Microscope to Give Energy-Filtered Images and Diffraction Patterns, and Electron Energy Loss Spectra" by R. F. Egerton, J. G. Philip, P. S. Turner and M. J. Whelan, Journal of Physics E: Scientific Instruments, Volume 8, 1033-1037 (1975). This reference describes a transmission electron microscope. The energy filter is a magnetic prism cooperative with an electrostatic mirror to transit the electrons twice through the prism. Direct imaging of surface elements is achieved with relatively high energy electrons (e.g. 80 kev). However, although the filtered energies of transmitted electrons are representative of elemental constituents, this instrument utilizes specially prepared thin film samples in transmission and is not intended to analyze solid surfaces.
An instrument for imaging secondary ions from surfaces is described in "Secondary Ions Microanalysis and Energy-Selecting Electron Microscopy" by R. Castaing, Electron Microscopv in Material Sciences. Academic Press (New York 1971) pages 103-161. This instrument accelerates very low energy (e.g. 10 ev) secondary ions from surfaces and performs mass and energy analysis while simultaneously forming two dimensional images.
A variety of electrostatic and magnetic electron lenses are known as described, for example, in the aforementioned text by Hren et al. One such magnetic lens is a single pole piece lens described in Hren et al. on Pages 68-69 as pancake and snorkel lenses. Further details of snorkel lenses are given in "Some Properties of Single Pole Piece Objective Electron Lenses" by S. M. Juma, M. A. A. Khaliq and F. H. Antar, Journal of Phvsics E: Scientific Instruments, Volume 16, 1063-1068 (1983). Single pole piece lenses apparently have not evolved to be useful in practical electron microscopy.
In view of the foregoing, a primary object of the present invention is to provide an electron microscope for two dimensional imaging of moderate energy (50 to 3000 ev) secondary electrons emitted from solid surfaces, where the emitted electrons are monochromatized in the process.
Another object is to provide a novel direct imaging microscope that is particularly useful for X-ray photoelectron chemical analysis of surfaces.
A further object is to provide a novel direct imaging microscope that is particularly useful for chemical mapping with Auger electrons.
Yet another object is to provide a monochromatic electron microscope having improved collection efficiency of electrons from a sample surface.